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- Author or Editor: Gary K. Grice x
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Abstract
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Abstract
During the evening of 24 May 1981, rainfall from a slow-moving, multicell thunderstorm exceeded 200 mm (8 in) in just 2 h over western sections of Austin, Texas. This intense precipitation, falling on previously saturated terrain, resulted in record flash flooding. The evolution of the large-scale meteorological setting associated with the flood is briefly examined. The presentation then focuses on the development and movement of several mesoscale convective systems (MCSs) over the southern Plains during the night preceding the flash flood. It is shown that two significant mesoscale convective outflow boundaries stalled and weakened in the Austin vicinity. The flash flood storm appears to have developed along a weak baroclinic zone left behind by these outflows. Additionally, the precursor MCSs primed the hydrologic setting in south central Texas as they produced 75–125 mm (3–5 in) rains. Upper-air soundings are considered and related to the anomalous (relative to other storms over Texas) movement of the flash flood thunderstorm.
Finally, the difficulties of dealing with this severe weather event are discussed from an operational perspective. Subtle interactions between a variety of mesoscale features appear to have occurred. The flash flood storm behaved much differently, for a brief period, than thunderstorms further to the west. How might these features and characteristics have been identified and monitored in real time? The final complication in the operational scenario was the issuance of a severe thunderstorm watch by the National Severe Storms Forecast Center. This led to a shift in emphasis in the statements being issued to the public from the flash flood danger to the apparently higher priority severe thunderstorm threat.
Abstract
During the evening of 24 May 1981, rainfall from a slow-moving, multicell thunderstorm exceeded 200 mm (8 in) in just 2 h over western sections of Austin, Texas. This intense precipitation, falling on previously saturated terrain, resulted in record flash flooding. The evolution of the large-scale meteorological setting associated with the flood is briefly examined. The presentation then focuses on the development and movement of several mesoscale convective systems (MCSs) over the southern Plains during the night preceding the flash flood. It is shown that two significant mesoscale convective outflow boundaries stalled and weakened in the Austin vicinity. The flash flood storm appears to have developed along a weak baroclinic zone left behind by these outflows. Additionally, the precursor MCSs primed the hydrologic setting in south central Texas as they produced 75–125 mm (3–5 in) rains. Upper-air soundings are considered and related to the anomalous (relative to other storms over Texas) movement of the flash flood thunderstorm.
Finally, the difficulties of dealing with this severe weather event are discussed from an operational perspective. Subtle interactions between a variety of mesoscale features appear to have occurred. The flash flood storm behaved much differently, for a brief period, than thunderstorms further to the west. How might these features and characteristics have been identified and monitored in real time? The final complication in the operational scenario was the issuance of a severe thunderstorm watch by the National Severe Storms Forecast Center. This led to a shift in emphasis in the statements being issued to the public from the flash flood danger to the apparently higher priority severe thunderstorm threat.
Abstract
Surface and upper air data for November 1974–February 1975 were examined in order to determine a method to locate and predict the movement of the wintertime arctic front over Alaska. Ten cases each of the front passing north and south over Fairbanks were studied, Frontal effects an Fairbanks were noted and methods to forecast the timing of the frontal passage at Fairbanks were determined. Movement of the arctic front over interior Alaska was found to be directly related to the location and movement of large low-pressure systems in the Gulf of Alaska. Only very light snowfall accompanied the front as it passed northward over Fairbanks. However, southward passage of the front accounted for 77% of the Fairbanks snowfall during the four mouth period. A marked temperature and moisture difference was evident between the two air masses separated by the arctic front. Periods of extremely low temperatures with clear skies and ice fog were observed when the front remained south of Fairbanks for several days. Cloudy skies and above normal temperatures predominated when the front was to the north. The 850 mb analysis using 2°C isotherm separation and 30 m height difference and the plotted Fairbanks radiosonde are suggested as aids for use in forecasting the movement of the arctic front across Fairbanks.
Abstract
Surface and upper air data for November 1974–February 1975 were examined in order to determine a method to locate and predict the movement of the wintertime arctic front over Alaska. Ten cases each of the front passing north and south over Fairbanks were studied, Frontal effects an Fairbanks were noted and methods to forecast the timing of the frontal passage at Fairbanks were determined. Movement of the arctic front over interior Alaska was found to be directly related to the location and movement of large low-pressure systems in the Gulf of Alaska. Only very light snowfall accompanied the front as it passed northward over Fairbanks. However, southward passage of the front accounted for 77% of the Fairbanks snowfall during the four mouth period. A marked temperature and moisture difference was evident between the two air masses separated by the arctic front. Periods of extremely low temperatures with clear skies and ice fog were observed when the front remained south of Fairbanks for several days. Cloudy skies and above normal temperatures predominated when the front was to the north. The 850 mb analysis using 2°C isotherm separation and 30 m height difference and the plotted Fairbanks radiosonde are suggested as aids for use in forecasting the movement of the arctic front across Fairbanks.
Abstract
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